A: Among other purposes, the mouth has been created for the Zikr of Allah Ta'aala, recitation of Qur'aan, recital of Durood, etc. It has not been created for the purpose mentioned in the question. Sucking of the private organ is an animalistic quality.

Usually during arousal, the organ secretes a fluid called Mazi. This fluid is Najis (impure) and Haraam to take in the mouth. This act must be avoided. Sperm (Mani) is also Najis (impure).